1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device by selectively controlling the growth of an epitaxial layer without a mask by irradiating a main surface of a substrate having a difference in level with light during the growth of a semiconductor layer by vapor phase epitaxy. The present invention also relates to a semiconductor device fabricated by the method.
2. Description of the Related Art
A light-emitting diode, a semiconductor laser, an FET (Field Effect Transistor), and a HEMT (High Electron Mobility Transistor), and the like are more and more miniaturized in size and shape so as to meet the demands in recent years for the devices to have higher efficiency and a higher operation rate. Under such conditions, a selective growth technique for growing a semiconductor by vapor phase epitaxy becomes important in order to form devices having a new structure or improve the performance of the devices.
In recent years, there has been an increasing interest in, as one of the selective growth techniques, a technique for controlling a crystal growth of a semiconductor by radiating light having a predetermined exposure pattern from the outside while the semiconductor layer is grown by vapor phase epitaxy so as to three-dimensionally selectively excite an epitaxial layer with light. If the light is radiated on the surface of the substrate during vapor phase epitaxy, the temperature of the surface of the crystal substrate rises, thereby accelerating the photochemical reaction of the semiconductor materials or the surface reaction to incorporate dopants. As a result, the epitaxial layer excited with light is formed.
In such a manner, if a light having a predetermined exposure pattern is radiated during the vapor phase epitaxy, differences between the irradiated portion and the unirradiated portion arise in the growth rate, the composition, and amount of dopant of the epitaxial layer. Therefore, the growth of the epitaxial layer can be selectively controlled without forming a mask on the substrate.
FIG. 11 is a vertical sectional view of an example of a conventional vapor phase epitaxy apparatus utilizing the selective growth technique with the light excitation described above. The vapor phase epitaxy apparatus is composed of an MOCVD (Metal Organic Chemical Vapor Deposition) apparatus provided with a light source for light excitation. The MOCVD apparatus is employed for growing a group III-V compound semiconductor by vapor phase epitaxy utilizing the thermal decomposition of group III organic gas and group V hydride gas.
In the vapor phase epitaxy apparatus, a flow channel 22 is provided within a reactor 21 made of quartz having a cylindrical shape. Gas introducing tubes 24 and 25 for introducing gas into the flow channel 22 are provided through an end wall 23 on one side of the reactor 21. Into the reactor 21, a susceptor 28 made of carbon for mounting a substrate 27, on which a semiconductor layer is grown by vapor phase epitaxy, is introduced from the other side of the reactor 21 in an axis direction. When the susceptor 28 is introduced into the reactor 21 so that the top face thereof on which the substrate 27 is mounted, it is exposed in the flow channel 22. In addition, a high-frequency coil 29 is wound around the outer side wall of the reactor 21.
The upper portion of the side wall of the reactor 21 has an opening 21a. The opening 21a works as a window for introducing light into the reactor 21 from the outside without causing distortion. The opening 21a does not have a path of cooling water for cooling the side wall of the reactor 21. Above the reactor 21, a laser device 30 for excitation is provided. In addition, also provided are: a mirror 31 for introducing laser light emitted from the laser device 30 to the opening 21a; a mask 32 for selective excitation for selectively transmitting the laser light reflected by the mirror 31; and an optical system apparatus 33 for focusing the laser light transmitted through the mask for selective excitation 32 on the substrate 27 in the reactor 21.
The following steps are required for selectively growing a semiconductor layer by using the vapor phase epitaxy apparatus with the light excitation. First, the substrate 27 is mounted on the susceptor 28 and then introduced at a predetermined position in the reactor 21. A current flows through the high-frequency coil 29 so as to heat the susceptor 28 and the substrate 27 to a predetermined temperature. Then, group III organic gas and group V hydride gas are introduced through the gas introducing tube 24 and the gas introducing tube 25, respectively, into the reactor 21 so as to flow through the flow channel 22. In the flow channel 22, the group III organic gas and the group V hydride gas are thermally decomposed, so that a group III-V compound semiconductor is grown on the surface of the substrate 27 by the vapor phase epitaxy. In this case, a carrier gas of hydrogen is introduced through a gas introducing tube 26 to flow outside of the flow channel 22 so as to prevent the residue from adhering to the inner surface of the side wall of the reactor 21.
When the laser light is emitted from the laser device 30 during the vapor phase epitaxy, the laser light is provided with a predetermined pattern at the mask 32 for selective excitation through the mirror 31. Then, the laser light is radiated on the surface of the substrate 27 through the optical system apparatus 33, thereby forming the epitaxial layer selectively excited with light.
In FIG. 12, a light-emitting diode with a current blocking layer fabricated by using the vapor phase epitaxy apparatus device is shown. Japanese Patent Application No. 4-36479 discloses the light-emitting diode.
The light-emitting diode includes a buffer layer 102, a first cladding layer 103, an active layer 104, a second cladding layer 105, a current blocking layer 106, and a current spreading layer 107 successively formed on a substrate 111 made of n-type GaAs. Trimethylindium (TMI)[In(CH.sub.3).sub.3 ], trimethylgallium (TMG)[Ga(CH.sub.3).sub.3 ], and trimethylaluminum (TMA)[Al(CH.sub.3).sub.3 ] as group III organic gases, arsine [ASH.sub.3 ] and phosphine [PH.sub.3 ] as group V hydride gases, and monosilane [SiH.sub.4 ] and dimethylzinc (DMZ)[Zn(CH.sub.3).sub.2 ] as dopants are appropriately supplied to the vapor phase epitaxy device. The buffer layer 102 is formed of n-type GaAs to have a thickness of 0.2 .mu.m. The first cladding layer 103 is formed of n-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P to have a thickness of 1.5 .mu.m. The active layer 104 is formed of non-doped (Al.sub.0.45 Ga.sub.0.55).sub.0.5 In.sub.0.5 P to have a thickness of 0.7 .mu.m. The second cladding layer 105 is formed of p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P to have a thickness of 1.5 .mu.m. Each of the layers is uniformly formed by the normal vapor phase epitaxy.
The current blocking layer 106 on the second cladding layer 105 is formed of (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P to have a thickness of 0.5 .mu.m. When the current blocking layer 106 is grown by the vapor phase epitaxy, the laser light emitted from the laser device 30 is incident thereon. In this case, the mask 32 for selective excitation has such a pattern that the laser light is radiated only to the central area of the substrate 111. When the laser light is radiated to the epitaxially growing surface of the current blocking layer 106, the decomposition of phosphine [PH.sub.3 ] supplied as group V hydride gas is accelerated, resulting in an increase in the effective V/III ratio of the growing surface or an increase in a percentage of Si atoms of monosilane supplied as a dopant to be incorporated with crystals. Therefore, the resultant current blocking layer 106 includes two regions having different compositions, that is, an n-type current blocking region 106a of the central area on which the laser light is radiated and a p-type current flowing region 106b of the remaining area on which the laser light is not radiated. Namely, a layer having different compositions can be formed by a single vapor phase epitaxy step without a mask.
Then, the current spreading layer 107 is formed on the current blocking layer 106 by the normal vapor phase epitaxy. The current spreading layer 107 is formed of a p-type Al.sub.0.7 Ga.sub.0.3 As to have a thickness of 5 .mu.m. Electrodes 108 and 109 are then formed: the electrode 108 is formed on the entire surface of the substrate 111 which is not in contact with the buffer layer 102; and the electrode 109 is selectively formed on a portion of the current spreading layer 107 only corresponding to the current blocking region 106a.
In the light-emitting diode fabricated in the manner described above, if the voltage is applied to the electrodes 108 and 109, the current flows through the diode avoiding the current blocking region 106a as indicated by the arrows of broken lines G shown in FIG. 12. This is because the current blocking region 106a of the current blocking layer 106 and the second cladding layer 105 form an n-p junction which is reversely biased. Therefore, since the injected current is diffused in a portion of the active layer 104 only corresponding to the current flowing region 106b, the light generated in the active layer 104 can be effectively emitted to the outside without being interrupted by the electrode 109, thereby making it possible to obtain high luminous efficiency.
With the vapor phase epitaxy apparatus described above, however, it is necessary to perform an alignment of the mask 32 for selective excitation and the optical system apparatus 33 with high accuracy since the laser light passing through the mask 32 outside the reactor 21 should be focused by the optical system apparatus 33 on the surface of the substrate 27 in the reactor 21 which is extremely far from the laser device 30. Moreover, a lens used for the optical system apparatus 33 should have a large numerical aperture and a low aberration.
For the reasons described above, the fabrication of a semiconductor device with a conventional vapor phase epitaxy apparatus has a problem in that an expensive optical system apparatus and the like with high accuracy are required, resulting in an increase in fabrication cost.